(1) Field of the Invention
The present invention relates to a device for charging a member to be charged, and an image forming apparatus using this charging device, and more particularly to a charging device used in an image forming electrophotographic system.
(2) Description of the Prior Art
In the past image forming electrophotographic systems have been widely used in copiers, laser beam printers, and other devices. As known well, in such electrophotographic systems, a corona discharge device is widely used for charging a photosensitive member. Generally, a corona discharge device comprises a fine wire and a shield electrode. A high voltage of about 4 to 5 kV is applied to the wire, and the photosensitive member is uniformly charged by the discharge taking place between the fine wire and shield electrode. For uniformity of charging of the photosensitive member, an electrode a grid may be disposed between the wire and the photosensitive member, and it is known as a Scorotron. At present, the Scorotron is very widely used.
However, the Scorotron requires a power source capable of applying a very high voltage of several kilovolts in order to stabilize the discharge. When discharging, moreover, ozone harmful to human health is massively generated. Accordingly, apparatus for treating the ozone is needed, or the photosensitive member may be deteriorated by the ozone.
Accordingly, methods and apparatus of very small ozone output have been proposed. They are intended to keep a conductive charging material in contact with the photosensitive member to be charged, and generate discharge between them, as well as the photosensitive member directly. As a result, the discharge for charging the photosensitive member may be kept to a necessary minimum limit, so that the ozone output can be reduced.
Known apparatus for charging directly by contacting with the photosensitive include a method using a conductive elastic roller as a charging member (Japanese Patent Publication No. 62-11343), and a method for using a fiber brush (Japanese Laid-open Patent No. 56-147159) are known among others. From the viewpoint of forming method of discharge electric field, a method of applying a direct-current voltage to a charging member (Japanese Laid-open Patent No. 58-194061), and a method of applying by superposing an alternating-current voltage and a direct-current voltage (U.S. Pat. No. 4,851,960) are known.
In the method using fiber brush, however, the contact state between the photosensitive member and fiber brush is unstable and charging is not uniform. Further, bristles of the fiber brush deteriorate or fall down due to aging effects, and charging is not stable.
By contrast, in the method using an elastic roller, as compared with the fiber brush, the contact state is relatively uniform, and aging effects are smaller. But, with the elastic roller, too, uneven charging due to surface roughness and uneven resistance of the roller also occurs. With respect to the voltage applied to the roller, the example of applying may be compared with the example of applying by superposing AC voltage and DC voltage. The charging uniformity has been found to be superior and the tolerance greater in the application method by superposing AC voltage and DC voltage. However, to apply AC voltage, a vibratory electric field is formed between the elastic roller and photosensitive member, which causes noise known as charging noise. This charging noise is the noise determined by the frequency of the applied voltage, and in particular it falls in the human audible frequency range (20 to 20000 Hz, especially 200 to 2000 Hz). To avoid this, therefore, it is necessary to lower (below 200 Hz) or raise (over 2000 Hz) the AC frequency. When the AC frequency is raised, the AC voltage attenuates extremely in the charging member and the efficiency is very poor. When the AC frequency is lowered, periodic charge unevenness occurs in the peripheral direction of the photosensitive member.
Supposing the AC frequency to be f (Hz) and the moving speed of the photosensitive member (called process speed) to be V.sub.p (mm/sec), periodic charge unevenness occurs at a pitch of V.sub.p /f mm in the peripheral direction of the photosensitive member. Its reason is explained below. First, the vibratory electric field gradually attenuates in the separating region of the charging member, and the surface potential of the photosensitive member converges at the superposed DC voltage. At this time, the applied AC frequency is finite, and at the end of charging (that is, when the surface potential of the photosensitive member converges), transfer of electric charge from the charging member to the photosensitive member and reverse transfer do not take place at the same time. Therefore, depending on the phase of the AC frequency at that time, charging is terminated when the final transfer or reverse transfer occurs. The phase of the AC voltage at the end of charging is the same regarding the axial position on the photosensitive member, but is different depending on the peripheral position. Thus, if the axial direction of photosensitive member is assumed to be in a lateral direction, charge unevenness in lateral stripes in synchronism with the AC frequency occurs. The pitch of the lateral stripes is V.sub.p /f (mm). When this pitch is larger than the pitch capable of developing by a developing device in an image forming apparatus, defective image occurs. To avoid this, therefore, it is necessary to increase the AC frequency f. For example, supposing an image forming apparatus having a printing speed of about four sheets of A4 format in vertical feed per minute (process speed 25 mm/s), the AC frequency is required to be 100 Hz or more.
In the case of an apparatus having a printing speed of about 30 sheets per minute (printing speed 190 mm/s), the AC frequency of over 750 Hz is required, but in this case the problem of charging noise occurs. In other words, by the AC frequency region in a range not to cause charging noise, the upper limit of the process speed of the image forming apparatus is determined. Accordingly, in the method of superposing DC voltage on AC voltage, it is hard to raise the printing speed.
Besides, an AC power source is large in volume and high in cost, which leads to larger size and higher cost of the image forming apparatus.
By contrast, when only the DC voltage is applied to the elastic roller, it is easy to raise the speed, the size is small and the cost is low, but, as mentioned above, the charging is uneven.